This research is directed to an important problem in ovarian biology, the underlying mechanisms through which the corpus luteum is disposed of after each estrous cycle. The P. I. has made the important observation that the steroidogenic luteal cells of the cow express class II major histocompatibility molecules, a property normally found only on professional antigen presenting immune/inflammatory cells. This unusual property of luteal cells and other observations have led the P. I. to the interesting and provocative hypothesis, that the luteal cells are the participants in a transient autoimmune response, which leads to rejection of the luteal tissue at the end of each cycle. Thus the Aims are: 1) characterize the expression of elements necessary for antigen presentation associated with class I and class II major histocompatibility molecules; 2) investigate the regulation of expression of specific proteins that comprise the antigen processing complex; 3) investigate the expression, functional significance and regulation of co-stimulatory molecules; 4) determine whether engagement between luteal cells and T lymphocytes is mediated through MHC class I and /or MHC class II-restricted interaction; 5) determine if alterations occur in the peptides presented by MHC class I and class II molecules expressed by functional vs. regressing corpora lutea. These studies are expected to reveal whether luteal cells have the requisite characteristics of antigen presenting cells, thus providing new information as test of a proposed transient autoimmune reaction during regression of the corpus luteum.